Method for producing di-organo alkaline-earth compounds

ABSTRACT

A process for the preparation of bisorganoalkaline earth metal compounds comprises reacting an organometallic compound R n M 2  with an alkaline earth metal salt M 1 X m .

The invention relates to a process for the preparation ofbisorganoalkaline earth metal compounds.

The invention further relates to a process for the polymerization ofanionically polymerizable monomers using a bisorganoalkaline earth metalcompound prepared by the aforementioned process as polymerizationinitiator.

The synthesis of bisorganoalkaline earth metal compounds by reactingbisorganomercury compounds with elemental calcium, strontium or bariumhas been described in U.S. Pat. No. 3,718,703. Although this synthesisroute produces the bisorganoalkaline earth metal compounds in goodyields, it is not desirable in view of the use of poisonous mercurycompounds.

The metallation of CH-acidic organic compounds with calcium, strontiumor barium in aprotic, polar solvents has been described in U.S. Pat.Nos. 3,965,080 and 4,012,336. In this way, it is possible, for example,to prepare dixanthenylbarium.

Russian Chemical Reviews, Vol. 50, 1981, p. 601-614, gives a review ofother possible syntheses and the use of organoalkaline earth metalcompounds in the anionic polymerization of unsaturated monomers. Some ofthe known syntheses for organoalkaline earth metal compounds are complexor produce the desired compounds in low yields or contaminated withby-products.

Burkey et al. in Organometallics 12 (1993), pages 1331-1337 describesolvent-free alkaline earth metal metallocenes solvated withtetrahydrofuran (THF) which are obtained by reacting potassiumcyclopentadienide with an alkaline earth metal iodide.

It is an object of the present invention to provide an economicalprocess for the preparation of bisorganoalkaline earth metal compoundsin high yields and high purity using readily available andeasy-to-handle starting compounds.

We have found that this object is achieved by a process for thepreparation of bisorganoalkaline earth metal compounds which comprisesreacting an organometallic compound R_(n)M² having a covalentmetal-carbon bond content with an alkaline earth metal salt M¹X_(m).

We have also found a process for the polymerization of anionicallypolymerizable monomers using, as polymerization initiator, abisorganoalkaline earth metal compound prepared by a process as claimedin any of claims 1 to 5.

The organometallic compounds which may be used for the novel process forthe preparation of bisorganoalkaline earth metal compounds are anycustomary organometallic compounds having a covalent metal-carbon bondcontent. Preference is given to organolithium, organosodium,organopotassium or organomagnesium compounds, in particularorganolithium or organomagnesium compounds, which are usually moresoluble. Suitable organic groups R are preferably σ-bonded hydrocarbonshaving from 1 to 25 carbon atoms, in particular C₁-C₂₅-alkyl, such asmethyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl,pentyl, hexyl, heptyl, octyl, nonyl, dodecyl, octadecyl, C₆-C₂₅-aryl,such as phenyl or substituted phenyl, such as 3,5-dimethylphenyl,p-t-butylphenyl, p-octylphenyl, p-dodecylphenyl, o-, m-, p-tolyl,biphenyl, naphthyl, aralkyl, such as benzyl, phenylethyl,2-phenylpropyl, 6-phenylhexyl, p-methylphenylethyl, p-t-amylbenzyl,C₃-C₁₂-alkenyl, such as vinyl, allyl, 3-butenyl., 4-hexenyl,C₃-C₁₂-cycloalkyl, such as cyclopropyl, cyclobutyl, cyclopentyl,cyclohexyl, cyclooctyl, C₃-C₁₂-cycloalkenyl, such as cyclohexenyl orcyclooctenyl. The organic groups R may carry functional groups which areinert to the metal-carbon bond. Examples thereof are trimethylsilyl,trimethylsiloxy, ether, dialkylamino or cycloalkylamino groups. Theinteger n is 1 where M² is an alkali metal, or 2 when M² is magnesium.Particularly preferred organometallic compounds are benzyllithium anddibenzylmagnesium. The organometallic compounds are often in the form ofadducts with solvents such as tetrahydrofuran, dioxane or complexingagents such as tetramethylethylenediamine or crown ethers. Thepreparation of the organometallic compounds is known per se. They can,for example, be prepared by reacting lithium or magnesium with theappropriate organohalides in organic solvents such as pentane, hexane ordiethyl ether. Suitable alkaline earth metals M¹ in the alkaline earthmetal salts M¹X_(m) are preferably calcium, strontium and barium. Theyare used in the form of their salts X as halides, amides, phosphides,alkyl oxides or aryl oxides, m being an integer 1 or 2 depending on thevalency of X. Preference is given to the readily availablebis(trimethylsilyl)amides or 2,4,6-tris(tert-butyl)phenoxides. Thealkaline earth metal salts can be obtained, for example, by reacting thealkaline earth metals with the corresponding hydrohalic acids, amines,phosphines, alkyl hydroxides or aryl hydroxides with the elimination ofhydrogen. Processes for the preparation of ether complexes by reactingcalcium, strontium and barium in ethereal solution saturated withammonia gas are described, for example, by S. R. Drake et al., MainGroup Chemistry 14) 1991, page 243, Polyhedron (12) 1993, page 2307-2311or Journal of the Chemical Society, Chemical Communications 1991, page517-519.

The process is preferably used for the preparation of organoalkalineearth metal compounds R₂M¹ having two identical organic groups R, i.e.homoleptic alkaline earth metal compounds.

When using mixed organometallic compounds, for examplebutyloctylmagnesium, or mixtures of different organometallic compounds,heteroleptic bisorganoalkaline earth metal compounds are also available.

The novel reaction of the organometallic compounds R_(n)M² with thealkaline earth metal salts M¹X_(m) is preferably carried out in anorganic solvent or solvent mixture in which the bisorganoalkaline earthmetal compounds produced have the lowest solubility product of allstarting materials and products. Suitable solvents or solvent componentsare aliphatic or aromatic, aprotic solvents, such as aliphatic orcycloaliphatic ethers, for example dimethyl ether, diethyl ether,dibutyl ether, diisopropyl ether, dioxane or tetrahydrofuran, aliphaticand cycloaliphatic hydrocarbons, such as pentane, hexane, octane,cyclohexane or cyclooctane, or aromatic hydrocarbons, such as toluene orethylbenzene.

The organometallic compound R_(n)M² is generally reacted with thealkaline earth metal salt M¹X_(m) in stoichiometrically equivalentamounts.

In a preferred embodiment, the alkaline earth metal salt is dissolved ina polar, aprotic solvent, to which a solution of the organometalliccompound is added dropwise, and the precipitate which forms is filteredoff and washed.

The resulting bisorganoalkaline earth metal compound can be furtherpurified where appropriate by the known methods of organometallicchemistry. For example, the bisorganoalkaline earth metal compound maybe dissolved in a solvent and admixed or covered with a layer of aprecipitant.

The temperature for the reaction is unimportant. It does of coursedepend on the melting points and boiling points of the solvents used,and also on the stability of the bisorganoalkaline earth metal compound.The reaction is usually carried out in the range from −80 to +50° C.,preferably in the range from 0 to 30° C.

The bisorganoalkaline earth metal compounds prepared by the novelprocess are suitable as polymerization initiators for anionicallypolymerizable monomers such as dienes, styrene, acrylates,methacrylates, acrylonitriles and vinyl chloride. They are particularlysuitable for the homo- and copolymerization of butadiene, isoprene andstyrene.

To improve the thermal stability (the bisorganoalkaline earth metalcompounds of lower alkyls are usually thermally unstable at roomtemperature) but also to improve the solubility, the bisorganoalkalineearth metal compounds may be reacted with 1,1-diphenylethylene. This isof particular importance if the polymerization is to be carried out inthe absence of polar solvents.

EXAMPLES

All operations were carried out under protective gas and in solventsfreshly distilled over sodium.

Barium bis(bistrimethylsilyl)amide*2 tetrahydrofuran was obtained by theprocedure in B. A. Vaartstra et al., Inorganic Chemistry 1991 (Vol. 30),page 121-125, by reacting barium with

Strontium bis(bistrimethylsilyl)amide*2 tetrahydrofuran was obtained bythe procedure in S. R. Drake et al., Journal of the Chemical Society,Chemical Communications 1991, page 517-519.

Strontium bis(2,4,6-tri-tert-butylphenoxide)*3 tetrahydrofuran wasobtained by the procedure of S. R. Drake et al., Polyhedron (11) 1992,page 1995-2007.

Dibenzylmagnesium*dioxane was obtained by the Grignard reaction ofmagnesium with benzyl bromide according to Y. Pocker et al., Journal ofthe American Chemical Society, 1968 (Vol. 90) page 6764.

Benzyllithium*tetramethylethylenediamine was prepared by the proceduregiven in B. J. Wakefield, Organolithium Methods, Academic Press 1988, onpage 38.

Preparation of dibenzylbarium Example 1

Preparation of dibenzylbarium by reacting bariumbis(bistrimethylsilyl)amide*2 tetrahydrofuran anddibenzylmagnesium*dioxane

A solution of 8.6 g (14.4 mmol) of barium bis(bistrimethylsilyl)amide*2THF in 50 ml of toluene/diethyl ether (4/1 vol.-%) was added dropwise at25° C. to a solution of 4.35 g (14.8 mmol) of dibenzylmagnesium*dioxanein 75 ml of toluene/diethyl ether (4/1 vol.-%) over the course of 10minutes, and the mixture was stirred for 14 hours. The yellowprecipitate formed was then filtered off and washed with 2×20 ml ofdiethyl ether. The residue was taken up in 30 ml of tetrahydrofuran andcovered with a layer of 60 ml of pentane. After 24 hours a firstprecipitate had formed. To complete crystallization the two phases weremixed, and the precipitate was filtered off and washed with 2×20 ml ofdiethyl ether. The dibenzylbarium was obtained in a yield of 1.82 g (5.7mmol), corresponding to 40%. ¹H-NMR characterization produced thefollowing results: 1.99 (s, 4H), 5.37 (t, 2H), 5.75 (d, 4H), 6.42 (t,4H). (All data in ppm, d8-THF solvent, down field signals of theincompletely deuterated solvent at 3.58 ppm).

Example 2

Preparation of dibenzylbarium by reacting bariumbis(bistrimethylsilyl)amide*2 tetrahydrofuran andbenzyllithium*tetramethylethylenediamine

A solution of 10.67 g (49.8 mmol) ofbenzyllithium*tetramethylethylenediamine in 150 ml of diethyl ether wasadded dropwise at 25° C. to a solution of 15 g (24.9 mmol) of bariumbis(bistrimethylsilyl)amide*2 THF in 100 ml of diethyl ether over thecourse of 20 minutes, and the mixture was stirred for 14 hours. Theorange precipitate formed was then filtered off and washed with 3×20 mlof diethyl ether. The dibenzylbarium was obtained in a yield of 7.44 g(23.3 mmol), corresponding to 93%.

¹H-NMR characterization produced the following results: 1.99 (s, 4H),5.37 (t, 2H), 5.75 (d, 4H), 6.42 (t, 4H). (All data in ppm, d8-THFsolvent, down field signals of the incompletely deuterated solvent at3.58 ppm).

Example 3

Preparation of dibenzylstrontium by reacting strontiumbis(bistrimethylsilyl)amide*2 tetrahydrofuran andbenzyllithium*tetramethylethylenediamine.

A solution of 0.38 g (1.77 mmol) ofbenzyllithium*tetramethylethylenediamine in 20 ml of diethyl ether wasadded dropwise at 25° C. to a solution of 0.50 g (0.90 mmol) ofstrontium bis(bistrimethylsilyl)amide*2 THF in 10 ml of diethyl etherover the course of 5 minutes. An orange precipitate quickly formed,which, after stirring for a further 14 hours, was filtered off andwashed with diethyl ether.

Example 4

Preparation of dibenzylstrontium by reacting strontiumbis(2,4,6-tri-tert-butylphenoxide)*3 tetrahydrofuran andbenzyllithium*tetramethylethylenediamine.

A solution of 1.39 g (6.5 mmol) ofbenzyllithium*tetramethylethylenediamine in 25 ml of toluene was addeddropwise at 25° C. to a solution of 2.51 g (3.02 mmol) of strontiumbis(2,4,6-tri-tert-butylphenoxide)*3 tetrahydrofuran in 15 ml of tolueneover the course of 10 minutes. An orange precipitate quickly formed,which, after stirring for a further 14 hours, was filtered off andwashed with 20 ml of toluene.

Example 5

Preparation of bis(1,1,3-triphenylpropyl)barium*2 tetrahydrofuran:

0.19 ml (1.07 mmol) of 1,1-diphenylethylene (DPE) was added at 25° C. toa solution of 150 mg (0.47 mmol) of dibenzylbarium (from Example 2) in20 ml of tetrahydrofuran, as a result of which the red coloration of thesolution quickly intensified. The mixture was stirred for a further 14hours and the solvent was removed under a high vacuum. To purify theproduct further, it was dissolved in 30 ml of benzene and filtered offfrom insoluble constituents. The filtrate was evaporated to drynessunder a high vacuum and the black-violet residue was washed with 40 mlof pentane. ¹H-NMR characterization produced the following results: 1.27(m, 8H, THF), 2.67 (m, 4H, CH2), 2.77 (m, 4H, CH2), 3.29 (m, 8H, THF),6.12 (t, 4H, para), 6.75 (t, 8H, meta), 6.85 (d, 8H, ortho), 7.10 (m,5H, 3-phenyl). (All data in ppm, C₆D₆ solvent, incompletely deuteratedsolvent at 7.15 ppm).

Example 6

0.21 ml (1.14 mmol) of 1,1-diphenylethylene (DPE) was added at 25° C. toa suspension of 170 mg (0.53 mmol) of dibenzylbarium in 20 ml of tolueneand 1 ml of tetrahydrofuran. The mixture was stirred for a further 14hours and a deep red solution was formed from the suspension.

Example 7

Example 6 was repeated except that ethylbenzene was used instead oftoluene.

Example 8

Polymerization of styrene:

A solution of 48 g (0.46 mol) of styrene in 450 g of cyclohexane wastitrated with an initiator solution as in Example 5 (0.03 mol/l ofbis(1,1,3-triphenylpropyl)barium*2 tetrahydrofuran in ethylbenzene)until there was a slight red coloration. 7.5 ml of the initiatorsolution were added to the mixture which was then polymerized for halfan hour at from 58 to 65° C. Polymerization was then terminated using 10ml of isopropanol. The polystyrene obtained had a number-averagemolecular weight Mn of 99,000 g/mol, a weight-average molecular weightof 145,000 g/mol and a dispersity D of 1.46 (gel permeationchromatography in tetrahydrofuran relative to a polystyrene standard).

We claim:
 1. A process for the preparation of bisorganoalkaline earthmetal compounds, which comprises reacting an organometallic compoundhaving a covalent metal-carbon bond content with an alkaline earth metalsalt.
 2. A process for the preparation of bisorganoalkaline earth metalcompounds as claimed in claim 1, which comprises reacting anorganometallic compound R_(n)M² with an alkaline earth metal saltM¹X_(m), where M¹ is Ca, Sr or Ba, M² is Li, Na, K or Mg, R is ametal-carbon-σ-bonded hydrocarbon having from 1 to 25 carbon atoms, X ishalide, amide, phosphide, alkyl oxide or aryl oxide and n and m areintegers 1 or 2, depending on the valency of M¹ or M² and X.
 3. Aprocess as claimed in claim 2, wherein the organic group R isC₁-C₂₅-alkyl, C₆-C₂₅-aryl, C₃-C₁₂-alkenyl, C₃-C₁₂-cycloalkyl orC₃-C₁₂-cycloalkenyl.
 4. A process as claimed in claim 2, wherein thebisorganoalkaline earth metal compound is a homoleptic organoalkalineearth metal compound R₂M¹ having two identical organic groups R.
 5. Aprocess as claimed in claim 1, which comprises reacting bariumbis(bistrimethylsilyl)amide or strontium bis(bistrimethylsilyl)amide orbarium bis(2,4,6-tri-tert-butylphenoxide) or strontiumbis(2,4,6-tri-tert-butylphenoxide) with benzyllithium ordibenzylmagnesium.
 6. A process as claimed in claim 1, which comprisescarrying out the reaction in an organic solvent or solvent mixture inwhich the bisorganoalkaline earth metal compound formed has the lowestsolubility product of all starting materials and products.
 7. A processas claimed in claim 1, which comprises carrying out the reaction indiethyl ether, toluene or a mixture thereof.